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NAME
erl_eval - The Erlang Meta Interpreter
DESCRIPTION
This module provides an interpreter for Erlang expressions. The expressions are in the abstract syntax as
returned by erl_parse, the Erlang parser, or io.
DATA TYPES
bindings() = [{name(), value()}]
binding_struct() = orddict:orddict()
A binding structure.
expression() = erl_parse:abstract_expr()
expressions() = [erl_parse:abstract_expr()]
As returned by erl_parse:parse_exprs/1 or io:parse_erl_exprs/2.
expression_list() = [expression()]
func_spec() = {Module :: module(), Function :: atom()}
| function()
lfun_eval_handler() =
fun((Name :: atom(),
Arguments :: expression_list(),
Bindings :: binding_struct()) ->
{value,
Value :: value(),
NewBindings :: binding_struct()})
lfun_value_handler() =
fun((Name :: atom(), Arguments :: [term()]) ->
Value :: value())
local_function_handler() = {value, lfun_value_handler()}
| {eval, lfun_eval_handler()}
| none
Further described below.
name() = term()
nlfun_handler() =
fun((FuncSpec :: func_spec(), Arguments :: [term()]) -> term())
non_local_function_handler() = {value, nlfun_handler()} | none
Further described below.
value() = term()
EXPORTS
exprs(Expressions, Bindings) -> {value, Value, NewBindings}
exprs(Expressions, Bindings, LocalFunctionHandler) ->
{value, Value, NewBindings}
exprs(Expressions,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler) ->
{value, Value, NewBindings}
Types:
Expressions = expressions()
Bindings = binding_struct()
LocalFunctionHandler = local_function_handler()
NonLocalFunctionHandler = non_local_function_handler()
Value = value()
NewBindings = binding_struct()
Evaluates Expressions with the set of bindings Bindings, where Expressions is a sequence of
expressions (in abstract syntax) of a type which may be returned by io:parse_erl_exprs/2. See
below for an explanation of how and when to use the arguments LocalFunctionHandler and
NonLocalFunctionHandler.
Returns {value, Value, NewBindings}
expr(Expression, Bindings) -> {value, Value, NewBindings}
expr(Expression, Bindings, LocalFunctionHandler) ->
{value, Value, NewBindings}
expr(Expression,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler) ->
{value, Value, NewBindings}
expr(Expression,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler,
ReturnFormat) ->
{value, Value, NewBindings} | Value
Types:
Expression = expression()
Bindings = binding_struct()
LocalFunctionHandler = local_function_handler()
NonLocalFunctionHandler = non_local_function_handler()
ReturnFormat = none | value
Value = value()
NewBindings = binding_struct()
Evaluates Expression with the set of bindings Bindings. Expression is an expression in abstract
syntax. See below for an explanation of how and when to use the arguments LocalFunctionHandler and
NonLocalFunctionHandler.
Returns {value, Value, NewBindings} by default. But if the ReturnFormat is value only the Value is
returned.
expr_list(ExpressionList, Bindings) -> {ValueList, NewBindings}
expr_list(ExpressionList, Bindings, LocalFunctionHandler) ->
{ValueList, NewBindings}
expr_list(ExpressionList,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler) ->
{ValueList, NewBindings}
Types:
ExpressionList = expression_list()
Bindings = binding_struct()
LocalFunctionHandler = local_function_handler()
NonLocalFunctionHandler = non_local_function_handler()
ValueList = [value()]
NewBindings = binding_struct()
Evaluates a list of expressions in parallel, using the same initial bindings for each expression.
Attempts are made to merge the bindings returned from each evaluation. This function is useful in
the LocalFunctionHandler. See below.
Returns {ValueList, NewBindings}.
new_bindings() -> binding_struct()
Returns an empty binding structure.
bindings(BindingStruct :: binding_struct()) -> bindings()
Returns the list of bindings contained in the binding structure.
binding(Name, BindingStruct) -> {value, value()} | unbound
Types:
Name = name()
BindingStruct = binding_struct()
Returns the binding of Name in BindingStruct.
add_binding(Name, Value, BindingStruct) -> binding_struct()
Types:
Name = name()
Value = value()
BindingStruct = binding_struct()
Adds the binding Name = Value to BindingStruct. Returns an updated binding structure.
del_binding(Name, BindingStruct) -> binding_struct()
Types:
Name = name()
BindingStruct = binding_struct()
Removes the binding of Name in BindingStruct. Returns an updated binding structure.
LOCAL FUNCTION HANDLER
During evaluation of a function, no calls can be made to local functions. An undefined function error
would be generated. However, the optional argument LocalFunctionHandler may be used to define a function
which is called when there is a call to a local function. The argument can have the following formats:
{value,Func}:
This defines a local function handler which is called with:
Func(Name, Arguments)
Name is the name of the local function (an atom) and Arguments is a list of the evaluated arguments.
The function handler returns the value of the local function. In this case, it is not possible to
access the current bindings. To signal an error, the function handler just calls exit/1 with a
suitable exit value.
{eval,Func}:
This defines a local function handler which is called with:
Func(Name, Arguments, Bindings)
Name is the name of the local function (an atom), Arguments is a list of the unevaluated arguments,
and Bindings are the current variable bindings. The function handler returns:
{value,Value,NewBindings}
Value is the value of the local function and NewBindings are the updated variable bindings. In this
case, the function handler must itself evaluate all the function arguments and manage the bindings.
To signal an error, the function handler just calls exit/1 with a suitable exit value.
none:
There is no local function handler.
NON-LOCAL FUNCTION HANDLER
The optional argument NonlocalFunctionHandler may be used to define a function which is called in the
following cases: a functional object (fun) is called; a built-in function is called; a function is called
using the M:F syntax, where M and F are atoms or expressions; an operator Op/A is called (this is handled
as a call to the function erlang:Op/A). Exceptions are calls to erlang:apply/2,3; neither of the function
handlers will be called for such calls. The argument can have the following formats:
{value,Func}:
This defines an nonlocal function handler which is called with:
Func(FuncSpec, Arguments)
FuncSpec is the name of the function on the form {Module,Function} or a fun, and Arguments is a list
of the evaluated arguments. The function handler returns the value of the function. To signal an
error, the function handler just calls exit/1 with a suitable exit value.
none:
There is no nonlocal function handler.
Note:
For calls such as erlang:apply(Fun, Args) or erlang:apply(Module, Function, Args) the call of the non-
local function handler corresponding to the call to erlang:apply/2,3 itself--Func({erlang, apply}, [Fun,
Args]) or Func({erlang, apply}, [Module, Function, Args])--will never take place. The non-local function
handler will however be called with the evaluated arguments of the call to erlang:apply/2,3: Func(Fun,
Args) or Func({Module, Function}, Args) (assuming that {Module, Function} is not {erlang, apply}).
Calls to functions defined by evaluating fun expressions "fun ... end" are also hidden from non-local
function handlers.
The nonlocal function handler argument is probably not used as frequently as the local function handler
argument. A possible use is to call exit/1 on calls to functions that for some reason are not allowed to
be called.
BUGS
Undocumented functions in erl_eval should not be used.